Optical power attenuation measurement is crucial to proper management of network communication systems. To this end, the Telecommunications Industry Association (TIA) and the International Electrotechnical Commission (IEC) established standard procedures for measuring the optical power attenuation associated with a device under test (DUT) using a light source and a power meter (referred to as the Light-Source Power-Meter (LSPM) approach) or an Optical Time Domain Reflectometry (OTDR) approach.
For the LSPM approach, the IEC 61280-4-1 Standard of the International Electrotechnical Commission (IEC) proposes different procedures for referencing an optical power attenuation measurement (OPLM) system. The referencing and measuring procedures were meant to standardize optical power attenuation measurements associated with a multitude of scenarios that are expected to occur in optical fiber cabling.
EF is a function (EF(r)) defined by international Standards, which characterizes the modal distribution of light in multimode optical fibers. It characterizes the near-field power distribution profile of light exiting (into air) the launch cord. It is defined as that proportion of the total exiting optical power which falls within a circle (i.e. “encircled”) of radius r at the end face of the fiber, where r is the radial distance from the optical center of the fiber core. Other methods of characterizing the modal distribution of light also exist.
When performing insertion loss and attenuation measurements of a multimode DUT, whether with the LSPM or the OTDR approach, the modal distribution of the test light must be carefully controlled in order to measure reproducible values of insertion loss or attenuation. If the modal distribution of the test light is not well controlled, “differential mode attenuation” may lead to unrepeatable and irreproducible measurement results.
In order to address this issue, test and measurement international Standards such as the Telecommunication Industry Association (TIA-526-14-B) and the International Electrotechnical Commission (IEC 61280-4-1) define requirements on the modal distribution of test light for performing measurements on multimode DUTs. For example, the IEC 61280-4-1 Standard provides for a target for the encircled flux function, EF(r) characterizing the modal distribution of test light launched into the multimode DUT and defines very tight tolerances on deviations from that target. More specifically, this Standard defines requirements based on lower and upper boundaries of EF values at four or five predefined radial values in the fiber core and for each of the two wavelengths, i.e. 850 and 1300 nm. These requirements apply to both the LSPM and the OTDR approaches.
When light is coupled into a multimode launch cord, depending on the coupling conditions and on the optical power density of the optical source, the coupling may result in light exiting the multimode launch cord being “underfilled” (i.e. too few modes are excited) or “overfilled” (i.e. too many modes are excited). The modal distribution of the test light launched into the multimode DUT need to be adjusted to comply with the EF requirements or any other suitable modal distribution requirement that may be defined by Standards. Various means for controlling the modal distribution of the test light exist in the art including that based on mandrel wrapping, i.e. the tight winding of a multimode optical fiber about a circular mandrel of a given diameter. Mandrel wrapping results in a preferential attenuation of the high-order modes corresponding to an initially overfilled condition. Launch conditions in compliance with a given set of modal distribution requirements (e.g. as defined in the IEC 61280-4-1 Standard) are said to be “mode conditioned”.
The Standards thus require the control of the modal distribution of test light launched into a multimode DUT in order to measure the optical power attenuation of the multimode DUT, whether with the LSPM or the OTDR approach. However, there is still room for improvement.